The present invention is directed to a novel method of removing iron contaminant formations commonly found in aqueous systems. The present method provides a means for dislodging and removing iron III scale formations which have formed in systems having iron and/or iron based alloy components in contact with the aqueous system.
The subject invention is directed to the use of a substantially non-toxic method of removing corrosion products of ferrous metals in contact with aqueous systems by effectively dislodging and removing these products from the system. The method described and claimed herein requires the use of at least one adjacent-paired or ortho-dihydroxy aromatic compound which further contains at least one electron withdrawing group pendant from the aromatic ring.
The present invention shall be described in terms of its utilization in conjunction with cooling water systems. However, the invention is not so limited but can be effectively used in conjunction with other aqueous systems which are in contact with iron base alloy material and, in particular, difficult to control systems which present conditions of elevated pH, high temperatures and/or high hardness levels such as boilers, heat exchangers and the like.
The dislodgement and/or removal of iron corrosion formation is essential in cooling water systems to cause the equipment to be free from obstruction and to produce an efficient system by maximizing flow rate and heat transfer. Iron(III) oxide scale is known to build-up in these systems and cause a marked decrease in their efficiency. The iron oxide scale is particularly troublesome because of its extremely low solubility in aqueous media and, therefore, deposits and builds up in the equipment to reduce the flow rate and impede the heat transfer. Thus, there is a need for additives which are capable of removing the iron scale and maintaining the system free of scale build-up. In order to effectively do this, the additive must be capable of dislodging and dissolving the iron oxide solids present in the system.
There are many means which have been suggested for iron scale removal. For a product to be useful under the conditions commonly found in cooling systems and the like, it must be capable of meeting the following combination of rigorous criteria:
1) dissolution of old rust scale; PA1 2) dissolution of solids of iron(III) oxides and iron(III) hydroxides which are materials normally insoluble in cooling water conditions of high pH and which normally is very hard water having high calcium and magnesium content; PA1 3) capable of controlling the formation of new rust scale as iron(III) over a sustained period; PA1 4) capable of removing the iron scale from the system without mere dislodgement of solids from its site of formation as such solids tend to be trapped at other locations of small dimension causing a clogging of the system; PA1 5) utilize material or compositions which are stable under the adverse conditions presented, such as high pH (6.5-9.5), high temperature (e.g. 100.degree.-175.degree. F.) and/or hardness associated with the presence of excess calcium, magnesium and carbonate ions; PA1 6) utilize material which is capable of remaining soluble under the adverse conditions presented; PA1 7) not being a source of corrosion or of accelerating the corrosion of the system; PA1 8) capable of complexing iron at low levels; PA1 9) capable of enhancing the performance of threshold scale inhibitors in the presence of iron; and PA1 10) capable of promoting the formation of protective oxide layers which render the metal surface passive to further corrosion.
It is readily seen that a means of achieving this combination of desired properties would find a high degree of acceptance in the control of iron scale in cooling water systems.
Scale prevention should be the primary goal for maintaining a clean system. However, inadvertent system upsets in pH, temperature, cycles of concentration, flow rate, etc., result in the formation of some iron scale which is not prevented by the normal maintenance dosage of conventional additives. Thus, there is a need for an additive which will remove iron oxide deposits which have been formed due to such upsets.
Classical methods for removing iron scale involve acid or mechanical cleaning. These are undesirable since they require costly shut down of the system being treated and lead to equipment degradation via corrosion and/or mechanical abrasion. A means for the removal of iron scale during normal operation of the cooling water system would present an improvement in economics, simplication of operation and minimize equipment degradation.
Polymeric additives have been used in an attempt to control iron scale in cooling water For example, U.S. Pat. No. 3,898,037 describes the dispersion of insoluble iron compounds with polymers of 2-acrylamido-2-methylpropanesulfonic acid. Sulfonated polymers have also been used for this purpose (Proc.-Int. Water Conf. Eng. Soc. West. Pa. 1978, 39, 299). These dispersants are capable of preventing iron oxide solids from depositing. They do not, however, dissolve iron(III) oxides and are not effective in removing hardened or crystalline deposits which are often present in cooling water systems.
Chelating agents have been used to prevent and remove iron oxide scales by sequestration Ethylenediaminetetraacetic acid has been used to remove iron oxide scale (U.S. Pat. No. 2,396,938), but it is effective only in the absence of excess calcium, a common condition encountered in cooling systems and the like. Its use can also lead to increased corrosion. Organophosphonates such as aminotrimethylenephosphonic acid (ATMP) have been shown to sequester iron(III) and thereby keep it soluble up to pH=10 (Dequest, Technical Bulletin 1-247, Monsanto, 1972). However, it is known that these organophosphonates will, in general, not dissolve iron oxide or iron hydroxide solids in water containing high concentrations of calcium ions at pH of 8 or above. These water conditions are typical for normal operation of a cooling water system and the like. In addition, these organophosphonates have been found to precipitate with high levels of calcium, precluding their use for iron-scale control in high hardness waters.
It is highly desired to have a means of dislodging and removing built up iron corrosion product and to disperse and dissolve them while further preventing such product formation of systems in which aqueous and iron or iron based alloy materials are in contact.